Direct Current Power Combiner

ABSTRACT

A circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple inductive elements. The inductive elements are adapted for operatively connecting respectively to the DC voltage inputs. Multiple switches connect respectively with the inductive elements. A controller is configured to switch the switches periodically at a frequency sufficiently high so that direct currents flowing through the inductive elements are substantially zero. A direct current voltage output is connected across one of the DC voltage inputs and a common reference to both the inputs and the output.

CROSS REFERENCE TO RELATED APPLICATIONS

The present applications benefits from U.S. application 61/050,268 filed on May 5, 2008 by the same inventor.

FIELD AND BACKGROUND 1. Field

The present invention relates to energy conversion and specifically to circuitry which combines multiple voltage inputs from serially connected direct current sources into a combined output.

2. Description of Related Art

Sunlight includes a spectrum of electromagnetic radiation emitted by the Sun onto the surface of the Earth. On the Earth, sunlight is filtered through the atmosphere, and the solar irradiance (Watts/meter square/nanometer W/m²/nm) is obvious as daylight when the Sun is above the horizon. The Earth receives a total solar irradiance determined by its cross section (π·R_(E) ², R_(E)=radius of the earth), but as the Earth rotates the solar energy is distributed across the entire surface area (4·π·R_(E) ²). The solar constant is the amount of incoming solar electromagnetic irradiance per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the solar rays. The solar constant is measured by satellite to be roughly 1366 watts per square meter (W/m²) or 1.366 W/m²/nm. Hence the average incoming solar irradiance, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar irradiance, is one-fourth the solar constant (approximately 0.342 W/m²/nm). At any given moment, the amount of solar irradiance received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.

The performance of a photovoltaic cell depends on the state of the atmosphere, the latitude and the orientation of the photovoltaic cell towards the Sun and on the electrical characteristics of the photovoltaic cell.

FIG. 1 shows schematically a graph of a solar irradiance 100 versus wavelength. Irradiance 100 is distributed around a peak wavelength at about 550 nanometers. FIG. 1 also shows schematically an absorption spectrum 102 of a typical solar photovoltaic (PV) cell with a given band-gap which allows only a portion of the solar irradiance to be converted into electrical power. The finite characteristic of the band-gap of the photovoltaic cell causes a substantial part of the sun's energy to remain unutilized. In order to improve photovoltaic efficiency, multiple junction cells have been designed which include multiple pn junctions. Solar irradiance not absorbed, because its energy is less than the band gap is transmitted to the next junction(s) with a smaller band gap and the transmitted radiation is preferentially absorbed and converted into electrical energy.

FIG. 2 shows the graph of solar irradiance 100 versus wavelength and three absorption spectra 202, 204 and 206 respectively of three photovoltaic junctions used in a single multi-junction cell designed to absorb different parts of the solar spectrum. The first photovoltaic junction having the largest band gap has an absorption spectrum 206, the second photovoltaic junction has an absorption spectrum 204, and the third photovoltaic junction which has the smallest band gap has an absorption spectrum 202. Combining the three pn junctions of photovoltaic junctions into a single multi-junction 30 cell increases the efficiency, theoretically to about 60% and practically today to above 40%.

FIG. 3 illustrates multiple multi-junction cells 30 connected in series. Each multi-junction cell 30 has three serially connected photovoltaic junctions 300, 302, and 304 which operate with three absorption spectra 206, 204 and 202 respectively. Multiple multi-junction cells 30 connected in series form a multi-spectral photovoltaic panel 3000 with output terminals 310 and 308.

FIG. 4 illustrates characteristic current-voltage curves of a single photovoltaic junction cell at different illumination levels. Curve 400 shows the maximum power point (MPP) for low light levels, curve 402 show the maximum power point MPP for higher light levels, and curve 404 shows the maximum power point MPP yet higher light levels assuming a constant temperature of the cell. As can be seen, at the different light levels the maximum power point is achieved at nearly identical voltages, but at different currents depending on the incident solar irradiance.

Reference is now made to conventional art in FIGS. 5a and 5b which shows a typical photovoltaic installation 50 operating in dark or partially shaded conditions and bright mode respectively. Bypass diodes 500 a-500 c are connected in parallel across photovoltaic panels 502 a-502 c respectively for instance according to IEC61730-2 solar safety standards (sec. 10.18). Photovoltaic panels 502 a-502 c are connected in series to form a serial string of photovoltaic panels. Referring to FIG. 5a , bypass diode 500 a provides a path 510 around photovoltaic panel 502 a during dark or partially shaded conditions. Current path 510 allows current to flow through bypass diode 500 a in the forward mode, preventing common thermal failures in photovoltaic panel 502 a like cell breakdown or hot spots. During forward mode, bypass diode 500 a preferably has low forward resistance to reduce the wasted power. FIG. 5b refers to normal operation or bright mode, forward current 512 will flow through photovoltaic panels 502 a-502 c while bypass diodes 500 a-500 c will operate in the reverse blocking mode. In reverse blocking mode, it is important that bypass diodes 500 a-500 c have the lowest high temperature reverse leakage current (I_(R)) to achieve the highest power generation efficiency for each photovoltaic panel 502 a-502 c.

BRIEF SUMMARY

According to the present invention there is provided a circuit including multiple direct current (DC) voltage inputs which including one or more shared terminals. A primary transformer winding includes a high voltage end and a low voltage end. The primary transformer winding has a tap or taps operatively connected to the shared terminals through a first switch. A secondary transformer winding includes a high voltage end and a low voltage end. The secondary transformer winding is electromagnetically coupled to the primary transformer winding. The secondary transformer winding has one or more taps operatively connected to the shared terminal(s) through a second switch. A direct current voltage output terminal connects the high voltage ends of the primary and secondary transformer windings. A low voltage direct current output terminal operatively connecting said low voltage ends of said primary and secondary transformer windings.

Diodes are typically connected in parallel with the first and second switches or the diodes are integrated with a transistor in a single package. The switches may be metal oxide semiconductor field effect transistor (MOSFET), junction field effect transistor (JFET), insulated gate field effect transistor (IGFET), n-channel field effect transistor, p-channel field effect transistor, silicon controlled rectifier (SCR) and/or bipolar junction transistor (BJT). A third switch optionally connects the low voltage end of the primary transformer winding to a common terminal; and a fourth switch optionally connects the low voltage end of the secondary transformer winding to the common terminal. Diodes are typically connected in parallel with the third switch and the fourth switch. Bypass diodes are operatively connected across the DC voltage inputs. Photovoltaic cells are optionally connected to the DC voltage inputs. The photovoltaic cells may be optimized for maximal optical absorption of different respective portions of the electromagnetic spectrum. The direct current voltage output terminal may be connected to a DC to DC converter.

According to the present invention there is provided a circuit including multiple direct current (DC) voltage inputs; multiple transformers including primary windings and secondary windings; multiple first switches connected respectively in series with the primary windings into a multiple of switched primary windings; and multiple second switches connected respectively in series with the secondary windings into multiple switched secondary windings. The switched secondary windings are parallel connected respectively with the switched primary windings by the DC voltage inputs. The switched secondary windings are adapted for connecting to a combined direct current power output combining the DC voltage inputs. The first and second switches are: metal oxide semiconductor field effect transistor (MOSFET), junction field effect transistor (JFET), insulated gate field effect transistor (IGFET), n-channel field effect transistor, p-channel field effect transistor, silicon controlled rectifier (SCR) and/or bipolar junction transistor (BJT).

According to the present invention there is provided a circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple tapped coils including respectively primary ends, secondary ends and taps. The taps are adapted for connecting individually to the DC voltage inputs. The first switches connect respectively in series with the tapped coils at the primary ends of the coils. The second switches connect respectively in series with the coils at the secondary ends of the coils. The taps serially connect respectively the first and second switches. A combined direct current power output is adapted for connecting between the tap of highest voltage and a reference to both the inputs and the output.

According to the present invention there is provided a circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple inductive elements. The inductive elements are adapted for operatively connecting respectively to the DC voltage inputs. Multiple switches connect respectively with the inductive elements. A controller is configured to switch the switches periodically. A direct current voltage output is connected across one of the DC voltage inputs and a reference to both the inputs and the output.

According to the present invention there is provided a method for combining direct current (DC) power. Multiple direct current (DC) voltage inputs are connected to respective inductive elements. Multiple switches are connected respectively with the inductive elements. The switches are switched periodically.

A direct current voltage output is combined by connecting across one of the DC voltage inputs and a reference common to both the DC voltage inputs and the direct current voltage output.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a graph illustrating typical spectra of solar irradiance and solar absorption of a single photovoltaic junction, according to conventional art.

FIG. 2 is a graph illustrating three different absorption spectra of three stacked photovoltaic junctions of a multi-junction photovoltaic cell, according to conventional art.

FIG. 3 illustrates serially connected multi-junction cells, according to conventional art.

FIG. 4 illustrates a current-voltage (TV) characteristic curve (arbitrary units) of a photovoltaic cell at three different illumination levels, according to conventional art.

FIGS. 5a and 5b illustrates a typical photovoltaic installation operating in during dark or partially shaded conditions and bright mode respectively, according to conventional art.

FIG. 6 illustrates a block diagram of photovoltaic installation with a power combiner according to an embodiment of the present invention.

FIG. 7 illustrates a power combiner circuit, according to an embodiment of the present invention.

FIG. 8 illustrates a power combiner circuit, according to another embodiment of the present invention.

FIG. 9 illustrates a photovoltaic system including multiple power combiners, according to an exemplary embodiment of the present invention.

FIG. 10 illustrates a flow diagram of a method, according to an embodiment of the present invention.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

By way of introduction, different embodiments of the present invention are directed toward compensating for current variations in multiple junctions cells or in serially connected photovoltaic cells and/or panels such as during partial shading while maximizing power gain, by avoiding the loss of power from one or more photovoltaic cells and/or panels shorted by the cells and/or panels respective bypass diode.

Reference is now made back to FIG. 3, which illustrates conventionally multiple multi-junction cells 30 connected in series, each with multiple serially connected photovoltaic junctions 300, 302, and 304. It is well known that the spectrum of solar irradiance on the Earth's surface is not a constant but varies according to many variables such as season, geographic location, time of day, altitude, atmospheric conditions and pollution. Hence, it becomes apparent that photovoltaic junctions 300, 302, and 304 sensitive to different spectrum bands may be absorbing a different amount of light depending on season, geographic location, time of day, altitude, atmospheric conditions and pollution. Since photovoltaic junctions 300, 302, and 304 are connected in series, the same current flows through all of the junctions. Thus, the best power point of serially connected photovoltaic junctions 300, 302, and 304 maximizes the overall power from photovoltaic junctions 300, 302, and 304, while each junction is typically producing a less than optimal amount of electrical power. On the other hand, a parallel connection of photovoltaic junctions and/or multi-junction cells, while allowing a better maximum power control for all photovoltaic junctions or multi-junction cells suffers among other possible power losses from an increase of ohmic power loss of the system since ohmic power loss is proportional to the square of the current. Furthermore, a parallel electrical connection of stacked pn junctions in a multi-junction cell is not particularly practical since multi-junction cells are typically stacked in a single production process and since the MPP voltage of each of these stacked pn junctions is different; the bandgap voltage for each pn junction is different.

The present invention in different embodiments may be applied to multiple photovoltaic cells and/or multi-junction photovoltaic cells connected in various series and parallel configurations with power converters/combiners to form a photovoltaic panel. Multiple series and parallel configurations of the photovoltaic panel and substrings within a panel with multiple power converters/combiners are used to form a photovoltaic installation. The present invention in further embodiments may be applied to other direct current power sources including batteries, fuel cells and direct current generators.

Embodiments of the present invention may be implemented by one skilled in the electronics arts using different inductive circuit elements such as transformers, auto-transformers, tapped coils, and/or multiple coils connected in serial and/or in parallel and these devices may be connected equivalently to construct the different embodiments of the present invention.

The terms “common”, “common terminal, “common reference” are used herein interchangeably referring to a reference common to both inputs and the output in the context of embodiments of the present invention. Typically, “common terminal” is ground, but the whole circuit may also be ungrounded. References to common terminal as ground are only illustrative and made for the reader's convenience.

Reference is now made to FIG. 6 which illustrates a block diagram of photovoltaic installation 600 with a power combiner 604 according to an embodiment of the present invention. A photovoltaic panel 60 has three photovoltaic cells 606 a-606 c connected in series. Photovoltaic cells 606 a-606 c are preferably multi-junction photovoltaic cells, photovoltaic cells or other direct current sources. An anode and cathode of a bypass diode D₁ connects across in parallel with photovoltaic cell 606 c at node F and node A respectively. An anode and cathode of a bypass diode D₂ connects across in parallel with photovoltaic cell 606 b at node A and node B respectively. An anode and cathode of a bypass diode D₃ connects across in parallel with photovoltaic cell 606 a at node B and node C respectively. Voltages V₁, V₂ and V₃ are the voltage outputs of photovoltaic cells 606 c, 606 b and 606 a respectively. Voltages V₁, V₂ and V₃ are applied to three voltage inputs of power combiner 604 as between nodes C & B, B & A and nodes A & F respectively. Power combiner 604 has a single output voltage V_(out).

Reference is now made to FIG. 7 which illustrates, according to an embodiment of the present invention, circuit details of DC power combiner 604. Three voltages V₁, V₂ and V₃ are input to power combiner 604 between nodes A and F, nodes B and A and nodes C and B respectively. Node B is on a “shared input terminal” of V₂ and V₃. Similarly, node A is on a “shared input terminal” of V₁ and V₂. One end of inductor L₁ connects to node C, the other end of inductor L₁ connects to one end of inductor L₃ to form node W. The other end of inductor L₃ connects to one end of inductor L₅ to form node X. The other end on inductor L₅ connects to the drain of MOSFET G₁ and the source of G₁ connects to node F (ground). One end of inductor L₂ connects to node C, the other end of inductor L₂ connects to one end of inductor L₄ to form node D. The other end of inductor L₄ connects to one end of inductor L₆ to form node E. The other end on inductor L₅ connects to the drain of MOSFET G₂ and the source of MOSFET G₂ connects node F (ground). The drain of MOSFET G₅ is connected to node W, the source of MOSFET G₅ connects to the source of MOSFET G₆. The drain of MOSFET G₆ connects to node D. The drain of MOSFET G₄ is connected to node X, the source of MOSFET G₄ connects to the source of MOSFET G₃. The drain of MOSFET G₃ connects to node E. The output voltage V_(out) of power combiner 604 is derived between nodes C and F (ground). A transformer core 601 is used to electromagnetically couple all inductors L₅, L₆, L₃, L₄, L₁ and L₂. The winding polarity of L₅, L₃ and L₁ is preferably opposite of the winding polarity of L₆, L₄ and L₂. The two inductors within each of the inductor pairs L₅-L₆, L₃-L₄ and L₁-L₂ typically have the same number of winding turns, although there can be a different number of turns to each of the inductor pairs (eg. L1 and L2, L3 and L4 and L5 and L6), to adjust the typical relative MPP voltage of each of the input voltages. Each of the three voltages V₁, V₂ and V₃ are applied across each of inductors L₅, L₃ and L₁ respectively with for instance a 50% duty cycle when switches G1, G4 and G5 are closed and switches G2, G3 and G6 are opened. Each of the three voltages V₁, V₂ and V₃ are applied across each of the inductors L₆, L₄ and L₂ respectively with typically a 50% duty cycle when switches G1, G4 and G5 are opened and switches G2, G3 and G6 are closed, thus completing a full switching cycle. The output voltage (V_(out)) of power combiner 604 is the sum of the input voltages V₁, V₂ and V₃. The input voltages V₁, V₂ and V₃ of power combiner 604 are forced by power combiner 604 to have the same ratio as the winding ratio of their inductor pair (L₅, L₆), (L₃, L₄) and (L₁, L₂) respectively; a result of applying control pulses to switches G₁-G₆ for instance with a 50% duty cycle. Switches G₁-G₆ are optionally metal oxide semiconductor field-effect transistors (MOSFET). Alternatively the switches can, in different embodiments of the invention, be a silicon controlled rectifier (SCR), insulated gate bipolar junction transistor (IGBT), bipolar junction transistor (BJT), field effect transistor (FET), junction field effect transistor (JFET), switching diode, mechanically operated single pole double pole switch (SPDT), SPDT electrical relay, SPDT reed relay, SPDT solid state relay, insulated gate field effect transistor (IGFET), DIAC, and TRIAC.

Reference is now made to FIG. 8 which illustrates, according to another embodiment of the present invention, an alternative circuit of DC power combiner 604. Three voltages V₁, V₂ and V₃ are input to power combiner 604 between nodes A & F, B & A and nodes C & B respectively. One end of inductor L₁ connects to node C, the other end of inductor L₁ connects to the drain of MOSFET G₁ the source of G₁ connects to node B. One end of inductor L₃ connects to node B, the other end of inductor L₃ connects to the drain of MOSFET G₃, the source of G₃ connects to node A. One end of inductor L₅ connects to node A, the other end of inductor L₅ connects to the drain of MOSFET G₅, the source of G₅ connects to node F (ground). One end of inductor L₂ connects to node C, the other end of inductor L₂ connects to the drain of MOSFET G₂, the source of G₂ connects to node B. One end of inductor L₄ connects to node B, the other end of inductor L₄ connects to the drain of MOSFET G₄, the source of G₄ connects to node A. One end of inductor L₆ connects to node A, the other end of inductor L₆ connects to the drain of MOSFET G₆, the source of G₆ connects to node F (ground). The output voltage V_(out) of power combiner 604 is derived between nodes C and F (ground). A transformer core 601 is used to electromagnetically couple all inductors L₅, L₆, L₃, L₄, L₁ and L₂. The winding polarity of L₅, L₃ and L₁ is preferably opposite of the winding polarity of L₆, L₄ and L₂ respectively. The two inductors within each of the inductor pairs (L₅ and L₆), (L₃ and L₄) and (L₁ and L₂) preferably have the same number of winding turns, although there can be a different number of turns to each of the inductor pairs, so as to adjust the typical relative MPP voltage of each of the input voltages.

Reference is now made to FIG. 9 which illustrates a photovoltaic system 90 including multiple power combiners 604, according to an exemplary embodiment of the present invention. Photovoltaic system 90 has multiple series strings 902 connected in parallel to the input of DC to AC converter 900. Series strings 902 have photovoltaic cells 904 a-904 c which are for instance multi-junction photovoltaic cells which have three voltage outputs V₁, V₂ and V₃ with three bypass diodes connected across each voltage output of photovoltaic cells 904 a-904 c. Connected to each photovoltaic cells 904 a-904 c is a three voltage input power combiner 604. Power combiner 604 has a single voltage output (V_(out)) which is applied across the input of DC to DC converters 92 a-92 c. The outputs of DC to DC converters 92 a-92 c are connected in series to form the input to DC to AC converter 900 and the output of multiple series strings 902.

Reference is now made to FIG. 10 which illustrates a method 10 according to an embodiment of the present invention. In step 11, DC voltage inputs are connected to inductive elements. In step 13, the inductive elements are switched at a high frequency dependent on the inductance values so that the inductive elements do not tend to “short” the input DC voltages. In step 15, a single output combines the DC inputs by connecting across typically the highest input voltage and a reference or ground common to both the DC inputs and the single output.

The definite articles “a”, “an” is used herein, such as “a multi-junction photovoltaic cell”, “a power combiner” or “a coil” have the meaning of “one or more multi-junction photovoltaic cells”, “one or more power combiners” or “one or more coils”.

Although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof. 

1-17. (canceled)
 18. A method comprising: connecting a first set of inductive elements, via a first set of switches, to a first input configured to receive a first voltage, and connecting a second set of inductive elements, via a second set of switches, to a second input configured to receive a second voltage; operating the first set of switches and the second set of switches based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and outputting, based on the operation of the first set of switches and the second set of switches, an output voltage.
 19. The method of claim 18, wherein the operating comprises operating the first set of switches based on a first respective duty cycle substantially equal to fifty percent.
 20. The method of claim 19, wherein the operating comprises operating the second set of switches based on a second respective duty cycle substantially equal to fifty percent.
 21. The method of claim 18, further comprising generating at least a portion of the first voltage or at least a portion of the second voltage using at least one photovoltaic cell.
 22. The method of claim 18, further comprising opening and closing the first set of switches and the second set of switches at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
 23. The method of claim 18, further comprising combining the first voltage with the second voltage.
 24. The method of claim 18, further comprising connecting a third set of inductive elements, via a third set of switches, to a third input configured to receive a third voltage, wherein the output voltage is a combination of the first voltage, the second voltage, and the third voltage.
 25. An apparatus comprising: a first set of inductive elements; a second set of inductive elements; a first set of switches; a second set of switches; a first input; a second input; and an output, wherein: the first set of inductive elements is connected via the first set of switches to the first input, the first input is configured to receive a first voltage, the second set of inductive elements is connected via the second set of switches to the second input, the second input is configured to receive a second voltage; the first set of switches and the second set of switches are configured to be operated based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and the output is configured to output, based on the operation of the first set of switches and the second set of switches, an output voltage.
 26. The apparatus of claim 25, wherein the first set of switches is configured to operate based on a first respective duty cycle substantially equal to fifty percent.
 27. The apparatus of claim 26, wherein the second set of switches is configured to operate based on a second respective duty cycle substantially equal to fifty percent.
 28. The apparatus of claim 25, wherein at least one of the first input or the second input is connected to at least one photovoltaic cell.
 29. The apparatus of claim 25, wherein the first set of switches and the second set of switches are configured to open and close at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
 30. The apparatus of claim 25, wherein the output voltage is a combination of the first voltage and the second voltage.
 31. The apparatus of claim 25, further comprising: a third set of inductive elements; a third set of switches; and a third input, wherein: the third set of switches is connected to the third input, the third input is configured to receive a third voltage, and the output voltage is a combination of the first voltage, the second voltage, and the third voltage.
 32. A system comprising: a voltage source configured to generate a first voltage and a second voltage; a first set of inductive elements; a second set of inductive elements; a first set of switches; a second set of switches; a first input; a second input; and an output, wherein: the first set of inductive elements is connected via the first set of switches to the first input, the first input is connected to the voltage source and configured to receive the first voltage, the second set of inductive elements is connected via the second set of switches to the second input, the second input is connected to the voltage source and configured to receive the second voltage; the first set of switches and the second set of switches are configured to operate based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and the output is configured to output, based on the operation of the first set of switches and the second set of switches, an output voltage.
 33. The system of claim 32, wherein the first set of switches is configured to operate based on a first respective duty cycle substantially equal to fifty percent.
 34. The system of claim 33, wherein the second set of switches is configured to operate based on a second respective duty cycle substantially equal to fifty percent.
 35. The system of claim 32, wherein the first set of switches and the second set of switches are configured to open and close at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
 36. The system of claim 32, wherein the output voltage is a combination of the first voltage and the second voltage.
 37. The system of claim 32, further comprising: a third set of inductive elements; a third set of switches; and a third input, wherein: the third set of switches is connected to the third input, the voltage source is configured to generate a third voltage, the third input is connected to the voltage source and configured to receive the third voltage, and the output voltage is a combination of the first voltage, the second voltage, and the third voltage. 